Sound reproducing system



July 23, 19350 J. H. HAMMOND, JR

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J. H. HAMMOND, JR SOUND REPRODUCING SYSTEM File d Aug. 23, 1930 6 Sheets-Sheet 6 396/1704 WIND/S J/kU/ID Patented July 23, 1935 omen STATE PATT FFHCL 14 Claims.

This invention relates to amplifying systems and more particularly to an improved method of. controlling the amount of amplification in accordance with the strength of the applied energy.

The invention relates particularly to a system of amplification in which a greater ratio of amplification or gain is produced for impressed signals of high intensity than for impressed signals of low intensity whereby the volume range of the signal is expanded.

One of the purposes of this invention is to provide a method by which the scratch and ground 7 noises ordinarily produced in the recording and reproduction of music or speech may be greatly reduced or eliminated.

This is accomplished by the use of an amplifying system which is capable of producing a greater ratio of amplification or gain for impressed signals of high intensity than for impressed signals of. low intensity. The present invention provides for controlling the operating characteristics of a space discharge means in accordance with the strength of the applied signal, so that the amplifying efiiciency of. the space discharge means is greater for strong signals than for weak signals.

This invention also provides the necessary means for maintaining quality and fidelity of reproduction and for preventing overloading of the variable-gainratio amplifier which might result in the production of undesired harmonics and other tones not originally present.

In orchestral conduction, dynamics refer to loudness of sounds, and dynamic range refers to the ratio of maximum intensity to the minimum intensity of sounds. The recording of orchestras bypresent methods necessarily imposes a limit upon the dynamic range which an orchestral conductor may utilize. n the one hand, the sound intensity produced by the orchestra cannot be so great as to overload the amplifying devices, causing a distortion of. the signals byproduct ion of new tones. On the other, it cannot be so weak as not to override stray disturbances; otherwise the response in the loud speaker will be inaudible because of the efiect of such noises.

It is obvious that there are limitations in the recording and reproduction of orchestra by disc and film, and principles of correction for such limitations are applicable in many fields. For example, in disc recording and reproduction, with a given number of threads per inch the maximum intensity permissible is limited by the necessity of avoiding overlapping of the sound tracks, and

the minimum intensity is limited by extraneous efiects such as needle scratch.

Dynamic multiplication refers to the process of increasing the dynamic range of electrical signals in an amplifier over that which would result with linear amplification. Without requiring the cooperation of special records, the dynamic multiplication is accomplished by suitable amplifier construction. Some of the important design requirements are:

l. The gain ratio of the amplifier must be an increasing function of the signal impressed upon it.

2. The dynamic control must be sufiiciently sluggish in operation such that the gain ratio 5 does not change with individual audio oscillations, but will be determined by an average effect over a considerable number of oscillations.

3. The dynamic multiplication must be such that detector action will not occur, within new tones being introduced which would not other wise be present.

4. Incorporated within the dynamic multiplier may be also devices to prevent all frequencies being treated alike dynamically, with the gainratio-frequency curve of different shape for different intensity values. Further, certain frequency ranges may beused to produce the stimulus for dynamic multiplication at the expense of the others.

5. The timing must be properly arranged whereby the gain ratio does not change too rapidly, so that one orchestral instrument does not modulate the output of another. For example, a dynamic multiplier system in which a blow on a kettle drum immediately emphasizes a piccolo sound is not to be desired. Further, however, it is of considerable importance that the gain ratio change more rapidly on crescendo that it does on decrescendo.

The invention also consists in certain 'new and original features of construction and combinations of parts hereinafter set forth and claimed.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization may be better understood by referring to the following description taken in'connection with the accompanying drawings forming a. part thereof, in which Fig. 1 illustrates diagrammatically the system as applied to the reproduction of the sound recrd of a talking moving picture projector and phonograph;

Fig. 2 is similar to Fig. 1 showing a modified form of the invention;

Fig. 3 shows curves of the characteristics of a typical vacuum tube;

Fig. 4 illustrates a portion of the curves shown in Fig. 8 on an enlarged scale;

Fig. 5 shows the amplification constant and internal impedance curves of a typical tube;

- Fig. 6 illustrates the gain ratio curves of the circuits shown in the accompanying figures;

Fig. 7 shows curves depicting the relation be tween input volts and grid bias;

Fig. 8 gives the gain ratio and output voltage curves for this type of circuit;

Fig. 9 illustrates the relation between input and output in decibels;

Fig. 10 shows the curve oi input signal voltage into the dynamic stage;

Fig. 11 illustrates the gain ratio curves of the dynamic stage;

Fig. 12 gives the output signal voltage curves; and a Fig. 13 shows a. modified form of rectifier.

Like reference characters denote like parts in the several figures of the drawings. I

In the following description and in the claims parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring more particularly to the drawings. Fig. 1 shows a standard talking moving picture projector l0 which is provided with the usual film ll. Light from a lamp i2 is focused by means oi a lens I3 through an aperture i4 and upon the sound record of the film H. The light then passes through an aperture II and is focused by means of a lens l8 upona photoelectric cell IT. This cell is connected through a battery 2| to the primary of a transformer 22. the secondary of which is connected to a potentiometer 2!. This potentiometer is in the input circuit of a space discharge means 24, the output circuit of which is connected to the primary of a trans former 25. one side of which is connected to a contact 26 of a double pole double throw switch 21. The upper blade of this switch is connected to the plate battery 2! of the space discharge means 24.

A standard phonograph mechanism ll is provided with the usual turntable 82 upon which rests a phonograph record IS. Mounted on the end of a movable arm S4 is a standard phonograph pick up device 35 provided with the usual needle 88. This pick up device is connected to a potentiometer II. the other side of which is connected to a second primary winding of the transformer 25. One side of this winding is connected to the lower blade of the switch 11. The movable contact of the potentiometer 81 is connected to a contact 88 of the switch 21.

The secondary of the transformer 25 is connected to the input circuit of a space discharge means M, the output circuit of which is connected to the primary of a transformer 42, the secondary of which is in the input circuit of a space discharge means Potentiometer 148 may be connected across the filament of device 48. In the grid return of the space discharge means 48 is a biasing battery 44 and a resistor 4'. the latter being shunted by a condenser 40. Across the resistor 48 is a rectifier 41 which is in series with the secondary of a transformer 42. The primary aooanoi of this transformer is included in the plate circult of a space discharge means 5|, the grid of which is connected by fixed or adjustable tap on a resistance 52, one side of which is connected by a blocking condenser 53 to the input circuit of the space discharge means 43. The other side of the resistance 52 is connected through a biasing battery .54 to the, filament of the space discharge means 5|.

The output circuit of the space discharge means 42 is connected to the primary of a transformer BI which is shunted by a resistance 56. The secondary of the transformer 55 is connected to the input circuit of a push pull amplifier, the output circuit of which is connected to the primary of a transformer B l, the secondary of which is connected to the moving coil of a loud speaker 82, the field winding 63 of whichis supplied with current from the battery 64.

In the operation of the form of the invention shown in Fig. 1 when it is desired to use the moving picture projector iii, the switch 21 is thrown to the right. The moving picture projector Ill is then operated in the usual manner, the light from the lamp l2 passing through the sound record on the iilm II. The light transmitted by this record is then received by the photoelectric cell H, the fluctuating currents from which pass through the primary of the transformer 22. The current from the secondary ofthis transformer then passes through the potentiometer 23 which may be manually regulated so that any desired amount of energy may be fed to the input circuit of the space discharge means 24, where it is amplified and fed to the primary of the transfurther 2!. The currents from the secondary of this transformer are then amplified by the space discharge means 4| and fed to the primary of the transformer 42. Y

The energy from the secondary of this transformer passes to the input circuit of the space discharge means 43, the bias of which is determined coniointly by the setting of the potentiometer ill, the voltage of the battery 44, and the potential drop in the resistance 45. Some of the energy from the secondary of the transformer 42 passes through the blocking condenser 58. which may be arranged to favor the passage of the higher frequencies to the resistance 52. which is connected to the grid of the space discharge means H, where the energy is amplified, passing through the primary of the transformer 48. The current in the secondary of this transformer is rectified by the rectifier 41 and flows through the resistance 45 in the direction of the arrow, thereby decreasing the bias on the space discharge means 48 as this current increases. This increases the gain ratio of the amplifier 43, thus increasing the. output of this device, which in turn increases the sound produced by the loud speaker 62. In this way as the energy input to the space discharge means 48 is increased the gain ratio of this device will also be increased so that the amplification will vary in accordance with the signal strength, being greater for strong signals than for weak ones. The object of the resistance It is to keep the frequencycharacteristics independent of the amplitude characteristies. The energy from the output circuit of the space discharge means 48 then passes through the transformer 55 to the push pull amplifier 51, where it is further amplified and fed through the transformer II to the loud speaker 82, where it reproduces the sound in the usual manner.

The range of sound produced by the loud speaker will be very much greater than the range of intensities of the recorded signals on either the film H or the phonograph record 33, so that in this manner a dynamic multiplication of the energy may be produced.

If it is desired to use the phonograph, the switch 21 is thrown to the left, thereby connecting the phonograph pick up 35 to the second primary winding of the transformer 25. The operation of the device from here on is similar to that just described in connection with the moving picture projector l0.

In the modified form of the invention shown in Fig. 2, the moving picture projector and the phonograph 3! together with their associated circuits are similar to those shown in Fig. 1 up to the secondary of the transformer 42 and are given similar reference numerals. This secondary is connected through a blocking condenser H and a variable tapped resistance 12 to the input circuit of the space discharge means 43, and is directly connected to the input circuit of the space discharge means One side of the secondary of the transformer 42 is connected through a biasing battery 13 to the filaments of the space discharge means 33 and 5|. The rest of the circuits associated with these devices are similar to those shown in Fig. 1 including the push pull amplifier 51 and loud speaker 62 and are given similar reference numerals.

The operation of this form of the invention differs from that of Fig. 1 primarily in permitting a greater voltage to be imposed on the space discharge means 5! than upon the space discharge means 43. It further provides, if desired, for actuating amplifier 51! from low frequency tones in preference to high tones. In either case, however, the tones actuating rectifier device 41 may be made to differ from those actuating amplifier $3 by suitable choice of blocking condenser, tapped resistor and of frequency characteristics of transformer 08.

If found desirable it would be possible to connect the input circuit of the space discharge means 5| to a separate secondary winding of the transformer 42, thereby making it unnecessary to use the blocking condenser H in the input circuit of the space discharge means 43.

The function of the resistance 56, which is across the primary of the transformer 55 is to keep the frequency characteristics of the latter independent of the amplitude characteristics.

It is thus seen that these systems produce a gainratio which is greater for strong signals than for weak signals and cause the range of signal strength in the output to be much greater than the range of signal strength in the input.

The maintenance of fidelity is accomplished chiefly by the use of resistance 56 which, for example, may be of the order of 10,000 ohms for normal operation. If, with maximum bias, the impedance of the space discharge means 43 is 70,000 ohms, and with minimum bias it is 15,000 ohms, then the impedance of space discharge means 03 and resistance 56 in parallel varies between limits 8750 and 6000, while if resistance 56 is omitted (that is made infinite) the impedance across primary of the transformer 55 would vary from 70,000 to 15,000. Thus a transformer designed for about 7000 ohms input impedance would have substantially constant transmission characteristics although the impedance of the space discharge means and gain ratio vary by a considerable factor. Resistance 50 could be made to operate inversely as the tube impedance to keep the parallel value more cbnstant if this is desired in a particular case.

It has been found that for avoidance of disturbances due to detector action, it is preferable to use a power type of rectifier so that the alteration of bias of the space discharge means 43 is substantially proportional to the input alternating voltage to the same space discharge means. This rectifier may take the form of .a permanent contact rectifier or a low powered space discharge means with separate source of filament heating. If a three electrode tube is used, the grid may be connected to the plate, or to the filament. Or, in special cases as shown in Fig. 13, the grid may be otherwise disposed of, as for example, to control the bias on the rectifying tube in such a manner as to limit its rectified output, and to prevent grid current from being drawn by the device 43.

For example, the connection of the grid to a tap on the resistance 05 (Fig. 13) limits the rectified output to prevent the change of bias due to resistance 45 exceeding a desired value, or to produce special output rectifier characteristics when several stages of dynamic amplification are cascaded.

The power rectifier device requires the use of the amplifying stage 5| to supply sufficient energy for its actuation, and to prevent distortion reaction which it would otherwise have upon the alternating voltage impressed upon the space discharge means 43. To avoid the tone currents reentering the grid of the space discharge means 43 by their presence in resistor 85, without circuit complications, it is necessary to use a large capacity condenser 46 which causes a time lag so that a time relation exists between the envelope of the incoming signals and the bias produced by the rectifier. But at the same time, the time lag on crescendo must not be so great as to permit high signal strength on space discharge means 43 before the bias has changed sufliciently. For good musical quality in orchestral reproduction it is desirable that the time lag be small on crescendo, but large on decrescendo. This condition is fulfilled by the use of the power type rectifier. from a small input signal strength, the rectifier M is a low impedance device, and quickly charges condenser 45 to alter the bias of space discharge means 43 before distortion due to overloading rectifier M can result. On decrescendo, however, when the .peak voltage across transformer 48 fails to exceed the direct voltage across resistance 55, the rectifier 01 cannot pass current in either direction, and is of infinite impedance, and the change of bias is controlled solely by the discharge of condenser 56 through resistor 35. Thus means is provided for rapid crescendo, and for a decrescendo that may be delayed in any manner desired.

Fig. 3 shows characteristic curves of .a typical receiving tube for three different plate voltages plotted in the usual manner showing milliamperes direct plate current for various grid-volts operating points. The point 0 shows the usual operating point for this tube in order that it may be operated in the linear region of its characteristics. Fig. 4, however, shows characteristics for the same tube at low plate current values with magnified scale. The range of plate meter, for example, used in Fig. 4 is l/ of a milliampere as against 4 milliamperes for the meter used in Fig. 3. Inspection of Fig. 4 shows that the so called linear region of operation is merely a matter of the manner in which the curves are plotted and that just as faithful amplification may be obtained,

Thus, on a rapid crescendo, starting for example, at an operating point at 12 volts bias and 90 volts on the plate as could be obtained at point 0 of Fig. 3 with 4 volts bias. For Fig. 4, however, the amount of voltage 5 swing permissible to impose on the grid circuit is much less than for Fig. 3, and moreover the tube characteristics at this point do not permit high amplification.

In Fig. 5 are shown the amplification constant and internal resistance of the tube for 90 volts plate voltage and difierent grid operating voltages. It will be noted that the amplification constant does change appreciably, whereas the internalresistance becomes high at the greater values of negative grid voltage. Both variations, however, are in such a manner that they produce similar results in causing decreased bias to produce increased amplification.

Fig. 6 shows the gain ratio obtainable at diffei'ent bias values and at 90 volts plate voltage for the tube the characteristics of which have been shown in Figs. 3, 4 and 5. This gain ratio, or the amplification of the stage, is computed from the,

formula in which ,u. and Re are amplification constant and internal impedance given by Fig. 5. "2 is the external output impedance of the tube and T. R.

is the transformer ratio in this example, assumed to be 3-1. These curves are shown for three values of Z", the external output im pedance.

Assuming unity power factor, it will be noted from the description above that the resistor shunting the primary of the output transfprmer limits the upper value of the external output impedance of. the tube and it is assumed in these computations that the output impedance and transformer step up ratio are not seriously dependent upon frequency. f,

Fig. 7 shows the input voltage to the amplifier Stage for different grid voltage operating values of the tube operated at 90 volts on the plate with approximately 6000 ohms loading. The heavy curved line shows the permissible alternating input voltage which will not cause distortion due to curvature of characteristics. The dotted line which is a continuation of this curve shows the permissible input voltage, provided the design is such that this permissible voltage is limited by.

glid current taken by the tube reacting back on circuits previous to the stage being described. The straight lines refer to voltage required to be impressed upon the auxiliary amplifier to shift the operating point from minus 12 volts to any specified less negative value. Three curves are shown for 5, 10 and 20 per cent rectifier efliciency which refers to the ratio of the power output to a resistance Rg equal to 250,000 ohms to the possible alternating power output of an amplifier tube of the characteristics shown in Fig. 3, operating at "0. For example, with 10% efliciency one volt impressed upon the auxiliary amplifier would.

change the bias on the dynamic multiplier tube from 12 volts to 5 ,4; volts. Whereas one volt impressed upon the dynamic multiplier at 12 volts bias would cause distortion, it will not cause such distortion when it has caused the change of bias to the new operating point 5%, volts. The adiustments as to rectifier efliciency may be readily made by change of the ratiooi voltage impressed upon the dynamic multiplier amplifier or it may be changed in a large number of other ways.

These curves show the importance of having adjustments suitably made that there will always be a factor of safety against distortion effects, especially in causing the bias to change rapidly on increased signals.

Fig. 8 shows the gain ratio and output voltage of the dynamic multiplier amplifier when the rectifier-is adjusted for 10% efiiciency and a bias of 12 volts is used with no impressed signal. The gain ratio for very weak signals is 8/10 and the corresponding plate current reading is .06 milliamperes. The gain ratio curve is fairly linear from 0-1 input volts, with the gain ratio increased by'a factor of 10 when full 1 volt is used. The output voltage of the amplifier is seen to be roughly proportional to the square of the input voltage.

Fig. 9 shows the relation between input and output of the amplifier on a decibel or logarithmic scale. In this diagram, 1 decibel is arbitrarily set to be equivalent to 1 volt. This diagram is entirely equivalent to the diagram of Fig. 8. This curve shows that for input range of 20 decibels the output range is 38 decibels, whereby the gain due to dynamic multiplication is 18 decibels. The amount of increase due to dynamic multiplication of course. depends upon the bias used for zero input signal strength, conveniently indicated by a meter reading.

In Figs-10, l1 and 12 are shown curves illustrating the operation of the dynamic multiplier in which abscissa represent time. In Fig. 10, the upper curve represents R. M. B. voltage due to the desired signal, and the lower line represents voltage due to extraneous noises such as needle scratch voltage. This noise level is assumed, for example, to be .08 volts. The Fig 11 curve shows the gain ratio of the dynamic stage. This curve is somewhat smoother than the curve of signal voltage which controls the. gain ratio curve, because of the timing features of the rectifier system which results in lag efi'ects, especially when the signal voltage is diminishing. In Fig. 12 the set of curves shows the output voltage, obtained by multiplying the input signal voltage, Fig. 10, by the gain ratio, Fig. 11, both for the desired signals and-the noise or stray signals. It will be noticed that during the interval between the two parts of the curve at which the desired signal exists the noise level is very low in comparison with the average sound level due to the signal, and that stray noise is amplified only when there is sufliciently large desired signal that the stray disturbances will not be noticeable. I

Although only a few of the various forms in which this invention may be embodied have been 55 shown herein, it is to be understood that the invention is not'limited to any specific construction, but might be embodied in various forms without departing from the spirit of the invention or the scope of the appended claims.

What is claimed is:

1. In the method of operating a system for reproducing sound from a sound record including an amplifier adapted to be operated in accordance with said sound record and to amplify the signals impressed thereon, the novel step which comprises operating said amplifier with a variable gain ratio which is controlled in accordance with the instantaneous intensity of the impressed signal in such manner that the gain ratio varies directly as said intensity.

2. A system of reproducing sound which comprises a sound record, a space discharge amplifier, means for operating said amplifier in accordance with said sound record, means for translating the '6 energy of said amplifier into compressional waves, and means controlled by the instantaneous intensity of signal for varying the amplification ratio of said amplifier directly as said intensity whereby the volume ratio of the reproduced signal is expanded with respect to the indicated volume ratio of said record.

3. In a sound reproducing system, a sound record, an amplifier controlled in accordance therewith and having a control element adapted to determine the .amplification ratio thereof, a resistance connected in series with said control element, 9. second amplifier operated in accordance with said sound record, a rectifier connected across said resistance and means for impressing the potential across said rectifier determined by.

the output potential in said second amplifier, said potential and said rectifier being so arranged that the potential of said control element is varied directly as the intensity of the impressed signal and causes the amplification ratio of said first amplifier to correspondingly vary.

4. A system of electrically reproducing sound from a phonograph record which comprises a phonograph pick-up, a space discharge amplifier operated thereby and means controlled by the intensity of signal applied to said amplifier for varying the amplification ratio thereof directly as said signal whereby the volume ratio of the reproduced signal is expanded with respect to the indicated volume ratio of said record.

5. A sound reproducing system comprisinga sound record, a space discharge amplifier associated therewith, means for varying the impedance of said amplifier in accordance with the applied signal in such manner that the volume ratio of the reproduced signal is expanded and means associated with the output circuit of said amplifier for preventing the change in impedance of said amplifier from reacting on said output circuit and causing distortion in said signal.

6. A sound reproducing system comprising a sound record, a variable impedance amplifier associatedtherewith and having an output circuit adapted to translate electrical energy into compressional sound waves and means associated with said output circuit for preventing the variations in impedance of said amplifier from being impressed upon said output circuit and causing distortion ofsaid sound waves.

'7. In a sound reproducing system, a sound record, a phonograph pick-up device therefor, a receiver, an amplifier between said pick-up device and receiver, a control circuit for varying the gain ratio of said amplifier, adjustable means between said pick-up device and amplifier for determining the amount of signal energy fed to said amplifier independently of the gain ratio, said controlcircuit being fed from a point between said adjustable means and pick-up device.

8. In a sound reproducing system, a sound record, a phonograph pick-up device therefor, a receiver, an amplifier between said'pick-up device and receiver, a control circuit for varying the gain ratio of said amplifier and fed by said pickup device and means for determining the amount of signal energy fed to said amplifier independently of the gain ratio.

9. In a sound reproducing system, a sound record, a phonographic pick-up device therefor, a receiver, an amplifier between said pick-up device and receiver, a control circuit for varying the gain ratio of said amplifier and fed by said pick-up device, adjustable means in said control circuit for determining the energy passed through the control circuit independently of the signal energy impressed upon said amplifier.

10. In a phonographic system, a source of audio selection frequency, a translating device, a dynamic modifier between said source of audio frequency and said translating device, a control path for varying the characteristics of said modifier and fed by said source of audio frequency, and adjustable means for determining the energy passing through said control circuit independently of the audio frequency energy impressed on said amplifier.

11. In a phonographic system, a source of audio selection frequency, a translating device, a dynamic modifier between said source of audio frequency and said translating device, a control path for varying the characteristics of said modifier and fed by said source of audio frequency, and adjustable means for determining the intensity of audio frequency energy fed to said modifier independently of the gain ratio.

12. In a phonographic system, a source of audio selection frequency, a translating device, a dynamic modifier between said source of audio frequency and said translating device, a control path for varying the characteristics of said modifier and fed by said source of audio frequency, said dynamic modifier being fed with audio signal energy from said source and with control energy from said path, and adjustable means for controlling one of said energies without afiecting the other of said energies.

13. In a sound record reproducing system, a pick-up device, an amplifier fed thereby, a sound propagating device fed by said amplifier, a circuit for controlling the gain ratio of said amplifier according to the signal energy, a rectifier in said circuit and means for increasing the impedance of said rectifier for high energy values.

14. In a sound reproducing system, a sound record, a pick-up device therefor, a dynamic amplifier fed by said pick-up device, a sound propagating device fed by said dynamic amplifier, a control circuit fed by said pick-up device for controlling the gain ratio of said amplifier in accordance with the average intensity of signal, a rectifier in said control circuit and a control element for increasing the impedance of said rectifier when said signal reaches high intensities.

JOHN HAYS HAIMMOND, JR. 

